I don't have a problem with Ilia and Deltans as they were conceived. I've read a lot of novels that have dug in much deeper into Deltan sexuality and culture. However, I do agree a little more exposition in the movie would be helpful. I can see how casual fans or even fans that haven't read novels and the background information might be WTF was all that about.
But I always liked when Star Trek presented different aliens with much different cultures. The Deltan physiology, of course, released strong pheromones that could overwhelm human beings (not just men, but women as well). They also had a much freer sexualized culture much different than human culture. The reason why they had to take an oath of celibacy and not engage in sexual activity with humans (again, it wasn't just men, but could affect women as well) was because humans and some other species can be overwhelmed by the pheromones. It doesn't mean human men are weak and unable to control themselves. It was simple biology. I don't see anything wrong with that. But yes, that should have been explained a bit more in the movie. I think they wanted to inject a bit of humor there but it looked more ridiculous as it was then funny.